A conventional seat back cushion for a vehicle seat is resiliently supported by means of a number of seat springs which are located between side frames of a seat back frame. The seat springs generally comprise zigzag springs or formed wire springs.
Quantities of bending of the seat springs locally vary depending on the magnitude of load imparted. The springs located in the upper and lower portions of the seat back frame are more bent than those located in the central portion of the seat back frame. Even in one spring, the central part of the spring is more bent than the right and left side parts of the spring. Thus, the quantities of bending of the springs are locally varied. This brings about deterioration of the stability and comfort of the seat when a person sits on the seat.
Japanese Patent Application Laid-Open No. 11-206506 discloses a structural arrangement where an auxiliary plate, which is forwardly and rearwardly movable, is provided in connection with the cushion plate.